Canon REBEL SL1 EOS 100D Instruction Manual page 6

Custom Function Settings *
0: Disable
1 : Enable
Improves the highlight detail. The dynamic range is expanded from
the standard 18% gray to bright highlights. The gradation between
the grays and highlights becomes smoother.
, With setting 1, the Auto Lighting Optimizer (p.130) is automatically set to
[Disable] and the setting cannot be changed.
With setting 1, noise (grainy image, banding, etc.) may become slightly
more pronounced than with setting 0.
[_] With setting 1, the settable range will be ISO 200 - ISO 12800 (up to ISO
6400 for movies).
Also, the <D+> icon will be displayed on the LCD monitor and in the
viewfinder when highlight tone priority is enabled.
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